Automatic transmissions for both two-wheel drive and four-wheel drive vehicles may have a closed-barrel design wherein automatic transmission fluid, hereinafter referred to as oil, is circulated through a hydraulic control circuit located in an upper case. Oil is pumped into the upper case for circulation through the valve body components, clutches and lubrication circuits and returned by means of drainback openings in the bottom of the upper case. Closed-barrel transmission cases tend to accumulate more transmission oil in the upper case region than prior art open barrel transmissions. Accumulation of oil in the upper case exacerbates problems relating to maintaining an adequate reservoir of oil for the oil pump.
An important problem addressed by oil pans for such transmissions is avoiding inadequate oil supply to the oil pump. The oil pump draws oil through an oil filter, which includes an inlet for drawing oil from the reservoir formed by the oil pan. If inadequate oil volume is contained in the reservoir, the inlet of the oil filter may draw air into the system and can cause cavitation or foaming of the oil.
In closed-barrel transmissions, restrictions in the oil flow paths in the upper case slow return of oil to the oil pan. Consequently, it is important that oil be allowed to flow freely from drainage points in the upper case.
It is an objective to maintain relatively high velocity oil flow in the oil pan so that sediment in the oil remains suspended until the oil is filtered. Prior art oil pans frequently include areas in the oil pan where oil flows slowly resulting in localized sedimentation.
When the vehicle is operated on hilly terrain, changes in the angular orientation of the transmission aggravate drainback problems. For example, when a vehicle proceeds up a hill, the oil flows to the rear of the transmission and returning oil is required to flow through the rear drainback opening. Drainback problems are also aggravated by cold temperatures. When the temperature of the transmission oil is reduced, its viscosity increases resulting in slower drainback.
When the transmission oil is at a low level, the probability of starvation of the oil pump increases. When the transmission is operating under normal conditions, less than ten percent (10%) of the transmission oil is retained in the oil pan.
A countervailing problem to the starvation problem is the problem of exceeding a maximum oil level of the transmission. An important consideration in transmission design is to maintain the oil level below the level of rotating elements in the transmission.
Transmission oil heated from ambient temperature by operation of the transmission undergoes thermal expansion. It is undesirable for thermal expansion to cause the maximum oil level of the transmission to be exceeded. It is an objective to lower the oil level as much as possible so that additional volume becomes available for thermal expansion. Ideally, adequate volumetric capacity is available in the transmission and in the transmission oil pan to avoid a transmission oil overfill condition when heated.
Prior art transmission oil pans addressed the starvation problem by providing an oil pan generally in the form of a large rectangular box-shaped member. The need for road clearance limits the depth of such oil pans. Large rectangular oil pans suffer from excessive shifting of the oil within the pan as the pan is tilted lengthwise. More oil is required to fill such pans adding expense without corresponding benefit. Large rectangular oil pans are also prone to deformation due to the lack of surface contour.
Oil de-aeration is an advantageous function of an oil pan wherein air bubbles in the oil are eliminated. The rate of oil de-aeration is a function of the volume of oil and the surface area of the reservoir in contact with air. To the extent that oil volume can be reduced and the oil level can be properly maintained while the surface area of oil in the pan in contact with air is increased, the rate of oil de-aeration can be improved.
Another function of an oil pan is to aid in cooling the oil. Oil pans having flat surfaces tend to have a lower surface area to volume relationship and do not offer enhanced cooling capabilities. Increasing the oil pan surface area exposed to ambient air results in an increase in the heat transfer rate of the oil pan.
It is frequently desirable to remove ferrous particles from the flowing transmission oil to avoid their recirculation through the transmission. Reducing oil flow velocity adjacent the magnetic filter improves the effectiveness of the magnetic filter by aiding in sedimentation in the area of the magnetic filter.
Magnetic filters of several designs have been proposed for filtration purposes. In U.S. Pat. No. 3,800,914 to Miyata, a contoured lubricant pan for use with an engine or transmission is disclosed which includes a rod-shaped filter disposed in a channel of a deep sump portion of the pan. The magnetic oil filter includes annular disks that are mounted on a rod. This and other prior art oil pans fail to provide a clear path from front and rear drainback openings to a sump.
Further, the prior art has failed to disclose the need for locating the oil sump at the cross-over point of the transmission wherein oil is maintained in an acceptable level under all reasonably foreseeable conditions. Reasonably foreseeable conditions are those conditions within the extremes of rear and front end inclination of the vehicle encountered when negotiating normal grades.
These and other problems are solved by the dual purpose automatic transmission oil pan of the present invention as summarized below.